1. Field of the Invention
The invention generally relates to a radio terminal for use in a TDMA time-division multiple access) communication system and more particularly to radio terminal with a function of fine adjusting the transmit timing by compensating for the internal delay time.
2. Description of the Prior Art
In a TDMA communication system, a reference or base station communicates with a plurality of (N) secondary or mobile stations (i.e., radio terminals) once in a time period called a frame. Each frame comprises N time slots assigned to respective radio terminals. In response to a reception of a down link signal to each of the radio terminals, the radio terminal has to synchronize a transmit signal therefrom so that the transmit signal is received by the reference station in a time slot (Ts) assigned to the radio terminal as shown in FIG. 1.
Specifically, in FIG. 1, it is assumed that if the reference station transmits a down link signal (hereinafter, referred to as a xe2x80x9cDL signalxe2x80x9d) in a time slot assigned to a specific radio station, the DL signal takes D/2 (sec) to reach the antenna of the radio station and further takes xcex1 (sec) to go through the receiver portion of the radio terminal. Here, D/2 is a propagation delay time between the reference station and the radio station and xcex1 is an internal delay time caused by components of the receiver portion of the radio station. It is also assumed that an up link (UL) signal transmitted from the radio station takes xcex2+D/2 (sec) to reach the reference station, where xcex2 is an internal delay time caused by components of the transmitter portion of the radio station. A TDMA method requires each of the radio terminals served by a reference station to adjust the transmission wait time T (sec) so as to satisfy the following equation:
Tf=T+D+xcex1+xcex2,
where Tf is a frame period or an interval between time slots assigned to each radio station and T is a time from the recognition of a DL signal to the beginning of a transmission operation or a transmission wait time. In other words, each radio station has to start a transmit operation T (=Tfxe2x88x92Dxe2x88x92(xcex1+xcex2)) sec after recognizing a DL signal.
The present invention relates to a method and apparatus for correcting the transmission timing by using the total internal delay time (xcex1+xcex2) in a radio terminal used in a TDMA communication system and to a radio terminal incorporating such a method and apparatus.
U.S. Pat. No. 5,363,373 issued Nov. 8, 1994 discloses a digital mobile station using presettable timeslot counter for compensating for propagation delay time. The mobile station receives a TDM signal from a cell site and detects a sync, a timeslot assignment signal and a signal indicating the propagation delay time (which corresponds to D in FIG. 1). From an assigned timeslot, a time interval from a detected frame sync to the time of transmission of a burst signal from the mobile station (Tf in FIG. 1) is determined. A pulse count corresponding to a subtraction (Tfxe2x88x92D) of the propagation delay time from the determined time interval is preset to a presettable counter. The counter starts counting when a frame sync is detected from the received TDM signal and generates a timing pulse when it reaches the preset pulse count to cause a burst signal to be transmitted. However, the delay time within the mobile station (i.e., xcex1+xcex2 in FIG. 1) is not considered in deciding the transmit timing.
U.S. Pat. No. 4,346,470 issued Aug. 24, 1982 discloses a method and apparatus for acquiring transmit synchronization at a secondary station with the periodic frame reference bursts from a reference station in a TDMA network. A propagation delay factor is first measured and then a propagation delay correction factor is measured by using the propagation delay factor in satellite communication. Since the internal delay time (xcex1+xcex2) is relatively shorter than the propagation delay time (D) in satellite communication, the internal delay time may be measured in a lower precision. The measurement of propagation delay factor or correction factor requires a transmission of burst. The synchronization scheme can not cope with changes with time in the characteristics of circuit elements constituting a radio terminal.
In some prior art radio terminals, the total internal delay time (xcex1+xcex2) is estimated or measured in a design or development stage and stored in a memory of each product or each manufactured radio terminal. The stored total internal delay time xcex1+xcex2 is used for calculation of the transmit timing in usual operation.
However, in such a radio communication system as requires a vary high precision of the transmit timing, the dispersion of the characteristics of circuit elements constituting each radio terminal can cause the internal delay times xcex1 and xcex2 to significantly vary with each radio terminal.
It is therefore an object of the invention to provide a radio terminal with the precision of transmit timing raised by actually measuring the total internal delay time (xcex1+xcex2) caused by the receiver and transmitter portions and fine adjusting the transmit timing with the measured total internal delay time.
According to an aspect of the invention, a radio terminal, for use in a time-division multiple access communication system, capable of adjusting a transmit timing by using a measured internal delay time is provided. The radio terminal comprises: an antenna; a duplexer coupled with the antenna and having an input terminal and an output terminal for using the antenna both for transmission and reception, respectively; a transmitter portion for converting a transmission signal into a transmission band signal to feed the input terminal; a receiver portion for converting a reception band signal from the output terminal into a demodulated signal; a reference pulse signal generator; a 1-of-2 switch, activated at latest at the same time as generating a reference pulse signal, for coupling the reference signal instead of the transmission signal with the transmitter portion; a synthesizer for causing the receiver portion to select a channel identical to the transmission band; an internal delay counter for counting clock pulses for a time period from the generation of a reference pulse signal to the reference signal returning as the demodulated signal to provide a number of counted clock pulses, Nd, corresponding to the time period (the time period being the internal delay time); and a time slot timing controller for providing a transmit timing signal after counting Ntxe2x88x92Nd clock pulses after a detection of a time slot assigned to the radio terminal from the demodulated signal, where Nt is a pulse count corresponding to a transmit timing at the antenna or a time period (Tfxe2x88x92D).
Providing an on/off switch between the antenna and the duplexer permit the radio terminal to measure the internal delay time without radiating radio waves.
The measurements of internal delay time regularly made permits the radio terminal to compensate for changes with time in the internal delay time due to the characteristics of circuit elements constituting the radio terminal.
According to another aspect of the invention, a method of determining a transmit timing for a radio terminal capable of adjusting the transmit timing by using a measured internal delay time is provided. The method comprises the steps of: detachably and electrically connecting a common terminal of an 1-of-2 switch to an antenna-side terminal of a duplexer of the radio terminal; connecting an antenna with one of two other terminals of the switch; connecting an with the other of the two other terminals of the switch; operating the switch such that the common terminal is connected with the impedance matching element; and determining the transmit timing in the same manner as in case of the radio station being alone.